In a typical aircraft, pneumatic (air) systems are used for a variety of purposes including but not limited to Environmental Control Systems (ECS), Engine Starting Systems, and De-Icing Systems. Power sources for the pneumatic systems generally initiate from the aircraft gas turbine engines or the auxiliary power unit (APU).
In a typical gas turbine engine, a compressor compresses air and passes that air along a primary flow path to a combustor where it is mixed with fuel and combusted. The combusted mixture expands and is passed to a turbine, which is forced to rotate due to the passing combusted mixture. When used on an aircraft, the primary purpose of this system is to provide propulsive force for the aircraft. A portion of the air compressed within the engine can be diverted from the primary flow path and be utilized in an air supply system. More specifically, a portion of the air can be diverted to a bleed inlet of a bleed air system. This compressed bleed air can be delivered to the airframe for various purposes, for example environmental control system (ECS). The bleed air flow from the engine can be over 900° F. (593° C.) and pressures over 350 psi.
The typical APU is a small turbine engine used to provide pneumatic, hydraulic and electrical power to the aircraft when the main gas turbine engines is not in operations either on the ground or in emergencies when the aircraft is in flight. Operation of the APU is similar to that of a typical gas turbine engine however the primary purpose of this system is to provide power to the electrical, hydraulic, and air supply systems.
The air supply system can include any number of systems including, but not limited to an engine starting system. An air turbine starter (ATS) is used to initiate the turbine engine rotation and is powered by the APU. The ATS is often mounted near the engines of the aircraft and the ATS can be coupled to a high pressure fluid source, such as compressed air, which impinges upon a turbine wheel in the ATS causing it to rotate at a relatively high rate of speed. The ATS includes an output shaft that is coupled to the turbine wheel and, perhaps via one or more gears, to the jet engine. The output shaft thus rotates with the turbine wheel. This rotation in turn causes the jet engine to begin rotating.